Flexible tibial sheath

ABSTRACT

A radially expandable sheath is provided having a substantially closed distal end with at least two sidewalls extending proximally therefrom and defining a central lumen. Each sidewall can have a substantially concave outer surface adapted to seat a graft member, and each side wall is at least partially separated by a longitudinally oriented slot that extends from a proximal end along a substantial length of each sidewall. The slot preferably terminates at a position just proximal to the distal end. The device can also include a sheath expander that is adapted to be disposed in the central lumen of the radially expandable sheath and that is configured to flex the sidewalls to radially expand the sheath so as to fix a graft member within a bone tunnel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/334,865 filed Oct. 26, 2016 and entitled “Flexible Tibial Sheath,”which is a continuation of U.S. patent application Ser. No. 14/593,080filed Jan. 9, 2015 and entitled “Flexible Tibial Sheath,” now U.S. Pat.No. 9,486,307, which is a continuation of U.S. patent application Ser.No. 13/788,872 filed Mar. 7, 2013 and entitled “Flexible Tibial Sheath,”now U.S. Pat. No. 8,956,410, which is a continuation of U.S. patentapplication Ser. No. 12/025,927 filed on Feb. 5, 2008 and entitled“Flexible Tibial Sheath,” now U.S. Pat. No. 8,435,293, which is adivisional of U.S. patent application Ser. No. 11/935,653 filed on Nov.6, 2007 and entitled “Flexible Tibial Sheath,” now U.S. Pat. No.7,699,893, which is a continuation of U.S. patent application Ser. No.10/608,899 filed on Jun. 27, 2003 and entitled “Flexible Tibial Sheath,”now U.S. Pat. No. 7,309,355, which are hereby incorporated by referencein their entireties.

FIELD OF THE INVENTION

The present invention relates to ligament fixation devices and methods,and more particularly to devices and methods for anchoring ligamentswithin a bone tunnel.

BACKGROUND OF THE INVENTION

Ligaments are tough bands of tissue which serve to connect the articularextremities of bones, or to support or retain organs in place within thebody. Ligaments are typically composed of coarse bundles of dense whitefibrous tissue which are disposed in a parallel or closely interlacedmanner, with the fibrous tissue being pliant and flexible, but notsignificantly extensible.

In many cases, ligaments are torn or ruptured as a result of accidentsor overexertion. Accordingly, various procedures have been developed torepair or replace such damaged ligaments. For example, in the humanknee, the anterior and posterior cruciate ligaments (i.e., the ACL andPCL) extend between the top end of the tibia and the bottom end of thefemur. The ACL and PCL cooperate, together with other ligaments and softtissue, to provide both static and dynamic stability to the knee. Often,the ACL is ruptured or torn as a result of, for example, asports-related injury. Consequently, various surgical procedures havebeen developed for reconstructing the ACL so as to restore normalfunction to the knee.

In many instances, the ACL may be reconstructed by replacing theruptured ACL with a graft ligament. More particularly, with suchprocedures, bone tunnels are typically formed in the top end of thetibia and the bottom end of the femur, with one end of the graftligament being positioned in the femoral tunnel and the other end of thegraft ligament being positioned in the tibial tunnel. The two ends ofthe graft ligament are anchored in place in various ways known in theart so that the graft ligament extends between the femur and the tibiain substantially the same way, and with substantially the same function,as the original ACL. This graft ligament then cooperates with thesurrounding anatomical structures so as to restore normal function tothe knee.

Despite the above-identified advances in the art, there remains a needfor a graft ligament anchor which is simple, easy to install, andinexpensive to manufacture, while providing secure, trouble-freeanchoring of the graft ligament.

SUMMARY OF THE INVENTION

The present invention generally provides a graft fixation device forfixing a graft member within a bone tunnel. In one embodiment, thedevice includes a bioabsorbable, radially expandable sheath having asubstantially closed distal end with at least two sidewalls extendingproximally therefrom and defining a central lumen. Each sidewall is atleast partially separated by a longitudinally oriented slot extendingfrom a proximal end along a substantial length of each sidewall andterminating at a position proximal to the distal end. The shape of thesidewalls can vary, but preferably each sidewall has a substantiallyconcave outer surface adapted to seat a graft member. Each sidewall canoptionally include surface features formed within the concave outersurface thereof. The device can also include a bioabsorbable sheathexpander, e.g., a tapered screw, adapted to be disposed in the centrallumen of the radially expandable sheath and configured to flex thesidewalls to radially expand the sheath so as to fix a graft memberwithin a bone tunnel. In an exemplary embodiment, the sheath expanderhas a largest diameter that is greater than a largest inner diameter ofthe radially expandable sheath in an unexpanded state.

The radially expandable sheath can have a variety of configurations. Inone embodiment, the distal portion of the radially expandable sheath,extending between a distal end of the longitudinally oriented slots anda distal end of the sheath, tapers to form a bullet-shaped distal tip.In another embodiment, at least two adjacent sidewalls are joined at aproximal end thereof by a stop member adapted to prevent over-insertionof the radially expandable sheath into a bone tunnel.

The configuration of the graft fixation device allows the device to beformed from a variety of materials, including materials having a lowelasticity. In an exemplary embodiment, the graft fixation device isformed from one or more polymers or copolymers formed from monomersselected from the group consisting of lactic acid, glycolic acid, andcaprolactone. In a more preferred embodiment, the material furtherincludes tricalcium phosphate.

In yet another embodiment, a graft fixation device for fixing a graftmember within a bone tunnel is provided. The device includes abioabsorbable, radially expandable sheath having a substantially closeddistal end with at least two sidewalls extending proximally therefromand defining a central lumen. Each sidewall has a substantially concaveouter surface adapted to seat a graft member, and each side wall is atleast partially separated by a longitudinally oriented slot extendingfrom a proximal end along a substantial length of each sidewall andterminating at a position proximal to the distal end.

In other aspects, a graft fixation kit for fixing a graft member withina bone tunnel is provided. The kit includes a bioabsorbable expandablesheath having proximal and distal ends with at least two sidewallsextending therebetween and defining a central lumen. Each sidewall is atleast partially separated by a longitudinally oriented slot extendingfrom the proximal end and terminating at a position just proximal to thedistal end, and each sidewall has an outer surface adapted to seat agraft member. The kit further includes a plurality of sheath expandersof varying sizes, each being disposable in the central lumen of theexpandable sheath and configured to flex the sidewalls to radiallyexpand the sheath so as to fix at least one graft member within a bonetunnel.

Methods for fixing a ligament graft in a bone tunnel are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention will bemore fully disclosed or rendered obvious by the following detaileddescription of the preferred embodiments of the invention, which are tobe considered together with the accompanying drawings wherein likenumbers refer to like parts, and further wherein:

FIG. 1 is a perspective view of a radially expandable sheath and asheath expander in accordance with one embodiment of the invention;

FIG. 2 is a perspective view of the radially expandable sheath shown inFIG. 1;

FIG. 3A is a side view of the radially expandable sheath shown in FIGS.1 and 2;

FIG. 3B is a cross-sectional view of the radially expandable sheathshown in FIG. 3A taken across line 3B-3B;

FIG. 4A is an illustration of a bone tunnel having four segments of aligament graft and a radially expandable sheath disposed therein in anunexpanded position; and

FIG. 4B is an illustration of the bone tunnel, ligament segments, andradially expandable sheath shown in FIG. 4A with the radially expandablesheath in the expanded position.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the present invention generally provides a radiallyexpandable sheath 10 for attaching a ligament graft to bone. In general,the expandable sheath has a substantially closed distal end with atleast two sidewalls (FIG. 1 illustrates four sidewalls 14, 16, 18, 20)extending proximally therefrom and defining an inner lumen 34. Eachsidewall 14, 16, 18, 20 can have a substantially concave outer surfaceadapted to seat a graft member, and each sidewall 14, 16, 18, 20 is atleast partially separated by a longitudinally oriented slot 22, 24, 26,28 that extends from a proximal end 10 a along a substantial length ofeach sidewall 14, 16, 18, 20. The slots 22, 24, 26, 28 preferablyterminate at a position just proximal to the distal end 10 b of thesheath 10. The device can also optionally include a sheath expander 12that is adapted to be disposed in the central lumen 34 of the radiallyexpandable sheath 10 and that is configured to flex the sidewalls 14,16, 18, 20 to radially expand the sheath 10 so as to fix a graft memberwithin a bone tunnel.

A person skilled in the art will appreciate that a variety of implantshaving virtually any configuration can be used to expand the expandablesheath, and that sheath expander 12 is merely one embodiment of animplant that can be used with the expandable sheath. Moreover, theexpandable sheath 10 can be used to attach a variety of materials tobone in a variety of medical procedures. Accordingly, the terms “graft”or “ligament graft,” as used herein, are intended to include any numberof natural and/or synthetic graft and/or tendon materials.

The expandable sheath 10 can have a variety of configurations, shapes,and sizes, but it should be adapted to expand within a bone tunnel toattach a ligament graft to bone. FIGS. 1-3B illustrate an exemplaryembodiment of an expandable sheath 10 having proximal and distal ends 10a, 10 b with four sidewalls 14, 16, 18, 20 extending therebetween. Whilefour sidewalls 14, 16, 18, 20 are shown, a person skilled in the artwill appreciate that the sheath 10 can have any number of sidewalls. Thesheath 10 should, however, have at least two sidewalls to allow thesheath 10 to expand upon insertion of a sheath expander therein. Eachsidewall 14, 16, 18, 20 preferably tapers from a proximal end 10 a ofthe sheath to a distal end 10 b of the sheath to form a sheath 10 havinga bullet-shaped profile. As a result, the proximal end 10 a of thesheath 10 defines the largest outer diameter or circumference Ds₁ of thesheath, and the distal end 10 b defines the smallest outer diameter orcircumference Ds₂ of the sheath, as shown in FIG. 3A. The sheath 10 alsoincludes an inner diameter x which is the largest at its proximal end 10a and decreases towards its distal end 10 b.

Each sidewall 14, 16, 18, 20 of the sheath 10 is preferably separatedfrom an adjacent sidewall by a longitudinally oriented slot 22, 24, 26,28 extending therebetween. Each slot 22, 24, 26, 28 can have the samelength ls, or alternatively the length of each slot 22, 24, 26, 28 canvary with respect to each other. In an exemplary embodiment, each slot22, 24, 26, 28 has the same length is and originates at the proximal end10 a of the sheath 10 and extends along a substantial length Ls of thesheath 10 to allow the sidewalls 14, 16, 18, 20 to flex with respect toeach other. Each slot 22, 24, 26, 28 preferably terminates at the sameposition P just proximal to the distal end 10 b of the sheath to providea slot-free distal tip 30. This termination point P defines the area atwhich each sidewall 14, 16, 18, 20 will bend during expansion of thesheath 10 by a sheath expander. Thus, while the termination point P canvary, the distance between the termination point P at the end of eachslot 22, 24, 26, 28 and the distal end 10 b of the sheath should besufficient to provide structural integrity to the device such that thesidewalls 14, 16, 18, 20 do not break apart from one another or from thedistal tip 30 during expansion.

Each sidewall 14, 16, 18, 20 of the sheath 10 can also have a variety ofshapes and sizes. In an exemplary embodiment, each sidewall 14, 16, 18,20 has a substantially concave outer surface that is adapted to seat agraft. The concave surface preferably extends along the length is ofeach sidewall 14, 16, 18, 20. The proximal-most portion of each sidewall14, 16, 18, 20, however, can include a flared region 32 (FIG. 2) toprovide an enlarged opening to the inner lumen 34 to facilitateinsertion of a sheath expander therein. Each sidewall 14, 16, 18, 20 canalso include one or more surface features 36 formed on the concavesurface to facilitate engagement of a graft between the sidewalls 14,16, 18, 20 and the bone tunnel when the sheath 10 is implanted. Thesurface features 36 can have a variety of configurations, and can beformed on all or a portion of one or more of the sidewalls 14, 16, 18,20. As shown in FIGS. 1-3A, the surface features 36 are formed from aseries of transversely-oriented ridges 36 formed along a substantiallyportion of each sidewall 14, 16, 18, 20. The ridges 36 are effective toengage or grip the graft to prevent sliding movement of the graft withrespect to the sheath 10. A person skilled in the art will appreciatethat the sheath 10 can include a variety of different features tofacilitate engagement between the sheath 10 and the graft.

Each sidewall 14, 16, 18, 20 can also include one or more longitudinalflexion regions to allow each sidewall 14, 16, 18, 20 to expand radiallyoutward upon insertion of a sheath expander into the lumen 34 of thesheath 10. FIG. 2 illustrates flexion regions 38 a, 38 b on sidewall 16.As shown, the flexion regions 38 a, 38 b are formed from substantiallyparallel edges that extend along the length Ls of the sheath 10 and thatdefine the concave shape of each sidewall 14, 16, 18, 20. The flexionregions can optionally be formed by thinning the material that formseach sidewall 14, 16, 18, 20 longitudinally in the region of desiredflexion, and in one embodiment, may be one or more longitudinal groovescut into each sidewall 14, 16, 18, 20. In use, the expansion of thesidewalls 14, 16, 18, 20 at the flexion regions 38 a, 38 b will retainthe graft material disposed within the sidewall 14, 16, 18, 20 by aninterference fit between the expanded sidewall 14, 16, 18, 20 and thebone tunnel wall.

While each sidewall 14, 16, 18, 20 is described as being separated by alongitudinally oriented slot, two or more sidewalls adjacent to oneanother can optionally include a stop member extending therebetween andadapted to prevent over-insertion of the sheath 10 into a bone tunnel.While the stop member can have a variety of configurations, FIGS. 1-3Billustrate an exemplary embodiment of a stop member 40 formed on theproximal-most end 10 a of the sheath 10 and extending between twoadjacent sidewalls, e.g., sidewalls 14 and 20, to connect the sidewalls14, 20. The stop member 40 can have a variety of configurations, and inone embodiment, as shown, is a tab-like protrusion that extends outwardfrom the circumference of the proximal end 10 a of the sheath 10. As aresult, the stop member 40 will abut the edge of a bone tunnel duringinsertion of the sheath 10 into the bone tunnel, thereby preventingover-insertion of the sheath 10.

While the stop member 40 connects two adjacent sidewalls, the stopmember 40 can optionally be adapted to break upon insertion of a sheathexpander into the sheath 10 to allow the sidewalls 14, 16, 18, 20 toexpand. To ensure that breakage occurs at the proper location, the stopmember 40 can include a weakened portion (not shown) formed at thedesired breakage point. A person skilled in the art will appreciate thata variety of techniques can be used to achieve the desired breakage.

The distal tip 30 of the sheath can also have a variety ofconfigurations, shapes and sizes. Since the distal tip 30 connects thefour sidewalls 14, 16, 18, 20 to one another to provide structuralintegrity to the sheath 10, the distal tip 30 is preferably slot-free,and also preferably does not include any surface features 36 formedthereon. While the shape of the distal tip 30 can vary, the distal tip30 preferably originates adjacent to the termination point P of eachlongitudinal slot 22, 24, 26, 8, and tapers toward the distal end 10 bof the sheath 10. The distal tip 30 can optionally include a flatteneddistal-most surface 42 (FIG. 3A) that joins a distal-most end of eachsidewall 14, 16, 18, 20 to one another. The edges (not shown) thatconnect the flattened surface 42 to each sidewall 14, 16, 18, 20 arepreferably rounded to form a substantially rounded distal tip 30. Thedistal tip 30 can also optionally include a bore 44 (FIG. 3B) extendingthrough the flattened surface 42 for receiving a guide wire tofacilitate implantation of the device. A person skilled in the art willappreciate that the distal tip 30 of the sheath 10 can have virtuallyany shape and size, and can optionally be substantially open or closed.

Referring back to FIG. 1, a sheath expander 12 can be used to expand theexpandable sheath 10 once the sheath 10 is inserted into a bone tunnel.While the sheath expander 12 can have virtually any configuration, FIG.1 illustrates an exemplary embodiment of a sheath expander 12 in theform of a tapered screw. The screw 12 includes a proximal end 12 adefining the largest diameter ds₁ of the screw, and a distal end 12 bdefining the smallest diameter ds₂ of the screw 12. Threads 50 areformed around the screw 12 and extend from the proximal end 12 a to thedistal end 12 b. In use, the screw 12 is adapted to expand theexpandable sheath 10, thus the largest diameter ds₂ of the screw 12 ispreferably larger than the largest inner diameter x (FIG. 1) of thesheath 10, and more preferably it is at least as large as the largestouter diameter Ds₁ of the sheath 10. The expander screw 12 alsopreferably includes a socket 52 formed in the proximal end 12 a thereoffor receiving a driver tool, such as a hex wrench, that is effective todrive the screw 12 into the sheath 10. The expander screw 12 can alsoinclude a lumen (not shown) extending therethrough for receiving a guidewire to facilitate insertion of the screw 12 into the sheath 10. Aspreviously stated, a person skilled in the art will appreciate thesheath expander 12 can have a variety of configurations, shapes, andsizes.

The expandable sheath 10 and sheath expander 12 can be used in a varietyof medical procedures, but they are preferably used to anchor ligamentswithin a bone tunnel. In an exemplary embodiment, the device or system10, 12 is used for tibial fixation of an anterior cruciate ligamentgraft. A bone tunnel is prepared in the patient's tibia, and the graftis separated into four tendon bundles, each of which is prepared by whipstitching a length of suture thereto. The graft is then passed throughthe tunnel and tensioned as necessary. While tensioning the graft, theexpandable sheath 10 is inserted into the opening of the bone tunnel,preferably by sliding the sheath 10 along a guide wire extending throughthe tunnel. A mallet or other impacting device can be used to gentlyadvance the sheath 10 into the tunnel. The stop member 40 will abut theopening of the tunnel when the sheath 10 is fully inserted. In thisposition, a graft bundle, which preferably includes four grafts 56, 58,60, 62, as shown in FIG. 4A, is preferably seated within each sidewall14, 16, 18, 20 of the expandable sheath 10. The sheath expander, e.g.,tapered expander screw 12, is then slowly inserted into the inner lumen34 of the sheath 10, using a driver tool, to expand the concavesidewalls 14, 16, 18, 20 of the sheath 10. As the sheath expander 12 isdriven into the sheath 10, the concave sidewalls 14, 16, 18, 20 of thesheath 10 will deform toward a circular geometry to conform with anouter diameter of the expander 12, and thus to compress each tendonbundle 56, 58, 60, 62 against the bone tunnel wall 70, as shown in FIG.4B. This will encourage rapid bony ingrowth into each bundle.

The sheath 10 of the present invention provides several advantages overprior art sheaths. In particular, the longitudinally oriented slots 22,24, 26, 28 reduce or eliminate the risk of cracking or splitting duringexpansion. While some prior art sheaths provide a weakened portionadapted to rupture or break upon insertion, the sheath can crack atunintended portions. The slots 22, 24, 26, 28 of the sheath 10 of thepresent invention eliminate the need for the sheath 10 to crack duringinsertion of the sheath expander 12 therein since the longitudinal slots22, 24, 26, 28 allow for expansion without cracking. The slots 22, 24,26, 28 also allow the sheath 10 to be formed from a material having arelatively low elasticity, e.g., a brittle material. Since the sheath 10does not need to be designed to break only at an intended location, theentire sheath 10 can be made from a brittle material. By way ofnon-limiting example, suitable materials include biocompatible,bioabsorbable polymers or copolymers formed of monomers selected fromthe group consisting of lactic acid, glycolic acid, and caprolactone. Ina further embodiment, the material can also include tricalciumphosphate. The sheath 10 can also be formed from bioabsorbable glassesand ceramics (possibly containing calcium phosphates and otherbiocompatible metal oxides (i.e., CaO)). The sheath 10 can also beformed from metals, or it can comprise combinations of metals,bioabsorbable ceramics, glasses or polymers. In an exemplary embodiment,the sheath is formed from polylactic acid (PLA), tricalcium phosphate(TCP), and optionally polyglycolic acid (PGA), and/or polycaprolactone(PCL). More preferably, the sheath is formed from one of the followingmaterials (all percentages are by volume unless otherwise indicated):

(1) 70% PLA+30% TCP

(2) 70% of a PGA/PLA mixture+30% TCP

(3) 70% of a PLA/PCL mixture+30% TCP

(4) 70% of a PGA/PCL/PLA mixture+30% TCP

Further, the longitudinal slots 22, 24, 26, 28 facilitate the use ofexpanders 12 having a wider variety of sizes, including the use ofexpanders 12 having an outer diameter ds₁ or circumference at least aslarge as the diameter Ds₁ or circumference of sheath 10. In this way, asingle sheath size may be stocked for a wide variety of procedures andintended bone tunnel sizes. In one embodiment, sheath 10 may be providedin a kit to surgeons in which a plurality of expanders having differentsizes are provided for use with a single size sheath.

It is to be understood that the present invention is by no means limitedto the particular constructions and methods herein disclosed and/orshown in the drawings, but also comprises any modifications orequivalents within the scope of the claims.

What is claimed is:
 1. A surgical method, comprising: inserting abiocompatible tubular elongate body into a bone while tensioning an endof each of a plurality of graft members outside of the bone, each of thegraft members being positioned between one of a plurality of sidewallsof the tubular elongate body and a wall of the bone; and inserting abiocompatible plug into the tubular elongate body, thereby causing aregion of each of the sidewalls to move radially outward and compressthe graft member between the sidewall and the wall of the bone such thateach of the graft members is fixed within the bone, wherein theplurality of sidewalls o the tubular elongate body do not break apartfrom one another throughout the radial movement.
 2. The method of claim1, further comprising advancing each of the plurality of graft membersinto the bone prior to inserting the tubular elongate body into thebone.
 3. The method of claim 1, further comprising advancing a guidewire into the bone, wherein inserting the tubular elongate body into thebone includes sliding the tubular elongate body over the guide wire. 4.The method of claim 1, wherein the tubular elongate body sheath has afeature at an end thereof that stops insertion of the tubular elongatebody into the bone after a predetermined length of the tubular elongatebody has been inserted into the bone.
 5. The method of claim 1, whereininserting the tubular elongate body into the bone includes advancing adistal portion of the tubular elongate body into the bone, and thesidewalls are attached to one another at a distal end of the tubularelongate body before the movement of the sidewalls and after each of thegraft members is fixed within the bone.
 6. The method of claim 1,wherein the tubular elongate body and the plug each bioabsorb into apatient's body having the bone therein.
 7. The method of claim 1,further comprising forming a bone tunnel in the bone, wherein insertingthe tubular elongate body into the bone includes inserting the tubularelongate body into the bone tunnel.
 8. The method of claim 1, wherein aninner lumen extends through the tubular elongate body along alongitudinal axis thereof, each of the sidewalls extends longitudinally,and inserting the plug into the tubular elongate body includes insertingthe plug into the inner lumen along the longitudinal axis.
 9. The methodof claim 8, wherein inner surfaces of each of the sidewalls form acontinuous wall that defines the inner lumen.
 10. The method of claim 1,wherein at least one protrusion formed on an exterior surface of each ofthe sidewalls engages the positioned graft member at least with thegraft members between the sidewalls and the wall of the bone.
 11. Themethod of claim 1, wherein the movement of the region of each of thesidewalls causes each of the sidewalls to deform from having a firstconcave shape to a different concave shape.
 12. A surgical method,comprising: inserting a biocompatible sheath into a bone whiletensioning an end of each of a plurality of graft members outside of thebone, each of the graft members being positioned between one of aplurality of concave sidewalls of the sheath and a wall of the bone; andinserting a biocompatible plug into the sheath, thereby causing a regionof each of the sidewalls to move radially outward and compress the graftmember between the sidewall and the wall of the bone such that each ofthe graft members is fixed within the bone, and change to a differentconcave shape, wherein the plurality of sidewalls of the sheath do notbreak apart from one another throughout the radially outward movement.13. The method of claim 12, further comprising advancing each of theplurality of graft members into the bone prior to inserting the sheathinto the bone.
 14. The method of claim 12, further comprising advancinga guide wire into the bone, wherein inserting the sheath into the boneincludes sliding the sheath over the guide wire.
 15. The method of claim12, wherein the sheath has a feature at an end thereof that stopsinsertion of the sheath into the bone after a predetermined length ofthe sheath has been inserted into the bone.
 16. The method of claim 12,wherein the sheath and the plug each bioabsorb into a patient's bodyhaving the bone therein.
 17. The method of claim 12, further comprisingforming a bone tunnel in the bone, wherein inserting the sheath into thebone includes inserting the sheath into the bone tunnel.
 18. The methodof claim 12, wherein an inner lumen extends through the sheath along alongitudinal axis thereof, each of the sidewalls extends longitudinally,and inserting the plug into the sheath includes inserting the plug intothe inner lumen along the longitudinal axis.
 19. The method of claim 18,wherein inner surfaces of each of the sidewalls form a continuous wallthat defines the inner lumen.